ORGANIC LETTERS 2012 Vol. 14, No. 12 3036–3039
HPLC-Assisted Automated Oligosaccharide Synthesis N. Vijaya Ganesh, Kohki Fujikawa, Yih Horng Tan, Keith J. Stine,* and Alexei V. Demchenko* Department of Chemistry and Biochemistry and the Center for Nanoscience, University of Missouri;St. Louis, One University Boulevard, St. Louis, Missouri 63121, United States
[email protected];
[email protected] Received April 25, 2012
ABSTRACT
A standard HPLC was adapted to polymer supported oligosaccharide synthesis. Solution-based reagents are delivered using a softwarecontrolled solvent delivery system. The reaction progress and completion can be monitored in real time using a standard UV detector. All steps of oligosaccharide assembly including loading, glycosylation, deprotection, and cleavage can be performed using this setup.
Solid-phase synthesis1,2 has been widely utilized in the routine preparation of oligopeptides3 and oligonucleotides.4 The use of polymer supports in oligosaccharide synthesis has also been reported.5 7 The use of these tech(1) Fruchtel, J. S.; Jung, G. Angew. Chem., Int. Ed. Engl. 1996, 35, 17–42. (2) Winter, M. In Combinatorial Peptide and Nonpeptide Libraries; Jung, G., Ed.; VCH: Weinheim, 1996, pp 465 510. (3) Merrifield, B. Br. Polym. J. 1984, 16, 173–178. (4) Solid-Phase Organic Synthesis; Toy, P. H., Lam, Y., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2012. (5) Schmidt, R. R.; Jonke, S.; Liu, K. In ACS Symposium Series (Frontiers in Modern Carbohydrate Chemistry); Demchenko, A. V., Ed.; Oxford University Press: 2007; Vol. 960, pp 209 237. (6) Seeberger, P. H. J. Carbohydr. Chem. 2002, 21, 613–643. (7) Seeberger, P. H.; Haase, W. C. Chem. Rev. 2000, 100, 4349–4393. (8) Boltje, T. J.; Kim, J. H.; Park, J.; Boons, G. J. Nat. Chem. 2010, 2, 552–557. (9) Parlato, M. C.; Kamat, M. N.; Wang, H.; Stine, K. J.; Demchenko, A. V. J. Org. Chem. 2008, 73, 1716–1725. (10) Kanie, O.; Ohtsuka, I.; Ako, T.; Daikoku, S.; Kanie, Y.; Kato, R. Angew. Chem., Int. Ed. 2006, 45, 3851–3854. (11) Jonke, S.; Liu, K.-g.; Schmidt, R. R. Chem.;Eur. J. 2006, 12, 1274–1290. (12) Crich, D.; Smith, M. J. Am. Chem. Soc. 2002, 124, 8867–8869. (13) Roberge, J. Y.; Beebe, X.; Danishefsky, S. J. J. Am. Chem. Soc. 1998, 120, 3915–3927. (14) Randolph, J. T.; McClure, K. F.; Danishefsky, S. J. J. Am. Chem. Soc. 1995, 117, 5712–5719. (15) Zheng, C.; Seeberger, P. H.; Danishefsky, S. J. Angew. Chem., Int. Ed. 1998, 37, 786–789. 10.1021/ol301105y r 2012 American Chemical Society Published on Web 05/30/2012
niques helps to expedite the synthesis of oligosaccharides and glycoconjugates8 16 by minimizing the necessity for purifying reaction intermediates and simplifying the removal of excess reagents that is usually achieved by filtration. To expedite solid-phase oligosaccharide synthesis, Seeberger et al. developed an automated approach, which was first accomplished by using a modified peptide synthesizer17 19 and very recently extended to “the first fully automated solid-phase oligosaccharide synthesizer”.20 Despite being a relatively new technique, it has already been applied to the synthesis of a variety of oligosaccharide sequences.20 23 Other recent enhancements of the supported synthesis of oligosaccharides include, but are not (16) Wu, X.; Grathwohl, M.; Schmidt, R. R. Angew. Chem., Int. Ed. 2002, 41, 4489–4493. (17) Plante, O. J.; Palmacci, E. R.; Seeberger, P. H. Science 2001, 291, 1523–1527. (18) Seeberger, P. H. Chem. Soc. Rev. 2008, 37, 19–28. (19) Plante, O. J.; Palmacci, E. R.; Seeberger, P. H. Adv. Carbohydr. Chem. Biochem. 2003, 58, 35–54. (20) Krock, L.; Esposito, D.; Castagner, B.; Wang, C.-C.; Bindschadler, P.; Seeberger, P. H. Chem. Sci. 2012, 3, 1617–1622. (21) Seeberger, P. H.; Werz, D. B. Nat. Rev. 2005, 4, 751–763. (22) Werz, D. B.; Castagner, B.; Seeberger, P. H. J. Am. Chem. Soc. 2007, 129, 2770–2771. (23) Walvoort, M. T. C.; van den Elst, H.; Plante, O. J.; Krock, L.; Seeberger, P. H.; Overkleeft, H. S.; van der Marel, G. A.; Codee, J. D. C. Angew. Chem., Int. Ed. 2012, 51, 4393–4396.
limited to, approaches employing fluorous tag,24,25 ionic liquid,26 28 nanoparticle,29 and nanoporous gold30 supports. As a part of an ongoing research effort, presented herein is the development of a new HPLC-based automated synthesis. To obtain clear evidence of the advantages of the new technology in comparison to the state-of-the-art polymersupported synthesis, we chose the most common approaches for all aspects of our synthesis:5,7 a solution-based trichloroacetimidate donor, a Tentagel resin-bound glycosyl acceptor attached via the anomeric center, TMSOTf as the activator/ promoter, CH2Cl2 as the reaction solvent, and Fmoc as the temporary hydroxyl protecting group.11,16,31 In accordance with the traditional manual synthesis, a glycosyl acceptor bound to the polymer beads is shaken with an excess of the solution phase glycosyl donor and promoter (Figure 1A). Periodic monitoring of the solution phase by TLC can provide information on whether the glycosyl donor still remains. The completeness of the coupling can be determined experimentally by performing test reactions, such as the Kaiser test,32 or by cleaving the product off the polymer support followed by characterization. Routinely, the coupling step is repeated two or three times using fresh reagents. Alternatively (or as necessary), the remaining hydroxyls can be capped to ensure that they will not interfere with subsequent steps. The new setup is based on an unmodified HPLC instrument, which is readily available in practically any synthetic or analytical laboratory. In brief, a chromatography column was packed with the preswelled polymer resin. The column was then connected to the HPLC system containing a pump (a three-head pump was used), a detector (a variable UV range detector), and a computer with standard HPLC-operating software installed (Figure 1B). The column was loaded with the glycosyl acceptor and purged with solvent, and then two separate solutions containing glycosyl donor and promoter were delivered concomitantly. After a relatively short reaction time, typically 30 60 min, the system was purged (washed) with solvent. At this time, the resin is loaded with the disaccharide derivative that can be either cleaved off of the polymer support or the oligosaccharide elongation can be continued via alternating deprotection/glycosylation steps. All steps can be monitored using a standard HPLC detection system set to record changes in the UV absorbance of the (24) Carrel, F. R.; Geyer, K.; Codee, J. D. C.; Seeberger, P. H. Org. Lett. 2007, 9, 2285–2288. (25) Jaipuri, F. A.; Pohl, N. L. Org. Biomol. Chem. 2008, 6, 2686– 2691. (26) Yerneni, C. K.; Pathak, V.; Pathak, A. K. J. Org. Chem. 2009, 74, 6307–6310. (27) Tran, A. T.; Burden, R.; Racys, D. T.; Galan, M. C. Chem. Commun. 2011, 47, 4526–4528. (28) Galan, M. C.; Corfield, A. P. Biochem. Soc. Trans. 2010, 38, 1368–1373. (29) Shimizu, H.; Sakamoto, M.; Nagahori, N.; Nishimura, S.-I. Tetrahedron 2007, 63, 2418–2425. (30) Pornsuriyasak, P.; Ranade, S. C.; Li, A.; Parlato, M. C.; Sims, C. R.; Shulga, O. V.; Stine, K. J.; Demchenko, A. V. Chem. Commun. 2009, 1834–1836. (31) Zhu, T.; Boons, G.-J. J. Am. Chem. Soc. 2000, 122, 10222–10223. (32) Kaiser, E.; Colescott, R. L.; Bossinger, C. D.; Cook, P. I. Anal. Biochem. 1970, 34, 595–598. Org. Lett., Vol. 14, No. 12, 2012
Figure 1. Comparison of the manual polymer-supported synthesis (A) with the new automated setup described here (B).
solution eluting off the column. A solution of reagents can be recirculated to reduce the amount required for each transformation. During our initial experimentation, the attachment of glycosyl acceptor precursor 1a (3.0 equiv based on the theoretical loading capacity of the resin) to TentaGel MBNH2 resin was accomplished using a conventional setup in the presence of EDC (3.0 equiv) and DMAP (1.0 equiv) in CH2Cl2. The Kaiser test32 was conducted on the resin to ensure complete loading (48 h). The resin was then treated with 10% trifluoroacetic acid in wet CH2Cl2 to afford polymer bound acceptor 2a (30 min, loading 0.29 mol/g). Next, resin 2a (190 mg, 55 μmol of the glycosyl acceptor) was swelled in CH2Cl2 for 4 16 h, loosely packed in the Omnifit SolventPlus chromatography column equipped with an adjustable end-piece (http://www.omnifit.com) and the column was integrated into the HPLC. The relative inefficiency of this protocol motivated us to use the HPLC experimental setup that has allowed us to expedite the initial loading. For instance, loading using a recirculating solution containing 1a (5 equiv), EDC (5 equiv), and DMAP (1 equiv) in CH2Cl2 (Pump B, 1.0 mL/min) was much more effective and the desired loading was achieved in 8 h. After that, the system was purged with CH2Cl2 for 10 min (Pump A, 2.0 mL/min flow rate) followed by detritylation that was accomplished using recirculating TFA/CH2Cl2/H2O (10/88/2, v/v/v, Pump C, 1.0 mL/min, 5 min) to afford 2a. The system was purged with CH2Cl2 for 10 min (Pump A, 2.0 mL/min flow rate). Reagent bottles, one containing a 39 mM solution of glycosyl donor 3a33 in CH2Cl2 and another one containing a 0.28 M solution of TMSOTf in CH2Cl2, were connected to inlets for pumps B and C, respectively. Pumps B/C were programmed to deliver the mixed solution of donor/promoter concomitantly in the ratio 4/1 (v/v) at the total flow rate 0.3 mL/min. After 60 min (18 mL total), pumps B and C were stopped and by this time ∼10 mol equiv of donor 3a (33) Colonna, B.; Harding, V. D.; Nepogodiev, S. A.; Raymo, F. M.; Spencer, N.; Stoddart, J. F. Chem.;Eur. J. 1998, 4, 1244–1254. 3037
(562 μmol) has passed through the column. It should be noted that the amount of reagents used for this initial study were chosen to mimic that used in the conventional polymer-supported synthesis. Apparently, the flow rate and the donor/promoter ratio can be easily adjusted by simple reprogramming of the pump operation. Since fresh reagents are delivered constantly, this eliminates the need for multiple reiterations of glycosylation, common in manual and other automated techniques. The column was purged with CH2Cl2 (pump A, 2.0 mL/min) for 10 min. The formation of disaccharide 4a was determined by cleaving off the sugar molecule from the resin using a 0.1 M solution of NaOCH3 in CH3OH/CH2Cl2 (5 mL, 1/1, v/v, pump C, 1.0 mL/min, 60 min) carried out by continuous recirculation followed by washing with CH3OH (2.0 mL/min) for 10 min. Standard (see the Supporting Information (SI)) neutralization and acetylation (Ac2O/pyridine) afforded product 4a in 98% yield (Table 1, entry 1). Encouraged by this result, we explored other donors using essentially the same experimental setup and reaction time. Thus, glycosidation of galactosyl donor 3b34 was equally effective and disaccharide 4b was isolated in 96% yield (Table 1, entry 2). Glycosidations using mannosyl and lactosyl donors 3c35 and 3d36 were somewhat less efficient, and the resulting oligosaccharides 4c and 4d were isolated in 78% and 67% yield, respectively (entries 3 and 4). Finally, glycosidations of glucosyl donors 3e and 3f equipped with an easily removable Fmoc protecting group at the C-6 and C-4 positions, afforded 4a in 95% and 92% yield, respectively (entries 5 and 6). Overall, these results were very indicative of the high efficiency of the new experimental setup based on HPLC, but that generalized reaction conditions (60 min) may not be universally applicable across all sugar series. This result implies that an extended reaction time would be beneficial for driving the glycosidation the less reactive 37 39 of mannosyl donor 3c to completion, as well as for the reaction with lactose imidate 3d. This called for further study, and we noted that the progress of the reaction could be determined from changes in the UV absorbance of the mixture eluting off the column (measured at 254 nm) in comparison to that measured for the initial 0.39 mmol solution of the donor that is entering the column and the expected total absorbance of the donor and TMSOTf (Figure 2A). Once no change is detected (the detector trace reaches a plateau), this can serve as an indication that the reaction had ended. Similar monitoring of the washing stage of the process can provide the desired information about its completion, the trace reaches the (34) Rio, S.; Beau, J.-M.; Jacquinet, J.-C. Carbohydr. Res. 1991, 219, 71–90. (35) Bien, F.; Ziegler, T. Tetrahedron: Asymmetry 1998, 9, 781–790. (36) Sandbhor, M. S.; Soya, N.; Albohy, A.; Zheng, R. B.; Cartmell, J.; Bundle, D. R.; Klassen, J. S.; Cairo, C. W. Biochemistry 2011, 50, 6753–6762. (37) Douglas, N. L.; Ley, S. V.; Lucking, U.; Warriner, S. L. J. Chem. Soc., Perkin Trans. 1 1998, 51–65. (38) Zhang, Z.; Ollmann, I. R.; Ye, X. S.; Wischnat, R.; Baasov, T.; Wong, C. H. J. Am. Chem. Soc. 1999, 121, 734–753. (39) Mydock, L. K.; Demchenko, A. V. Org. Lett. 2008, 10, 2103– 2106. 3038
Table 1. Exploratory Comparative Study of the Coupling Efficiency of Glycosyl Donors 3a f with the Polymer-Bound Acceptor 2a
baseline corresponding to the standard absorbance of neat CH2Cl2. Real-time monitoring has helped us to optimize the reaction time and reduce the amount of reagents required.
Figure 2. Idealized sketches for the UV detector monitoring: (A) glycosylation washing steps; (B) Fmoc deprotection washing. Org. Lett., Vol. 14, No. 12, 2012
Table 2. Experimental Data for the HPLC-Assisted Synthesis of Pentasaccharide 8
Thus, the optimized reaction times for glycosylation of acceptor 2a using a 0.3 mL/min combined flow rate for the donor and promoter solutions (4/1, v/v) are as follows: 3a, e,f (30 min), 3b (25 min), 3c (70 min), and 3d (85 min). Typical washing times for all reactions at 1.0 mL/min of CH2Cl2 are 5 10 min. We also found that slight lowering the concentration of glycosyl donors or performing partial recirculation of the “used” solution by connecting the column outlet with the pump intake (recirculation) is nearly as efficient as using the standard “fresh” 39 mmol solution. However, UV monitoring of experiments wherein reagents are continuously recirculated is cumbersome. Having optimized model glycosylations we decided to undertake the synthesis of an oligosaccharide chain using the UV detection system. This synthesis began with glycosyl acceptor 2b and donor 3e equipped with the temporary Fmoc protecting group. The standard glycosylation protocol (60 min, step 1, Table 2) followed by a 10-min wash (step 2) led to the formation of disaccharide 5a. Subsequent
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deprotection of the Fmoc group in 5a was carried out as follows: purge with DMF for 1 min (2.0 mL/min) and then pass a 20% solution of piperidine in DMF (0.5 mL/min). The release of the dibenzofulvene piperidine adduct was monitored by the UV detector at 312 nm. Based on the realtime Fmoc deprotection curve (Figure 2B), we were able to reduce the reaction time to 5 min (step 3). This transition was followed by a 10-min wash with CH2Cl2 (step 2) to afford disaccharide acceptor 5b. The synthesis of trisaccharide 6 was continued using polymerbound disaccharide acceptor 5b and donor 3e via sequential execution of steps 1 2 3 2 (glycosylate wash deprotect wash). The same sequence was repeated to obtain tetrasaccharide 7. Finally, pentasaccharide was obtained by steps 1 and 2 followed by the cleavage step 4 using a recirculating 0.1 M solution of NaOCH3 in CH3OH/CH2Cl2 followed by acetylation to afford compound 8 in 62% yield. A comparative yield of pentasaccharide 8 was obtained using the manual setup, but the experimental time was significantly longer. In conclusion, we developed a new technology for automated oligosaccharide synthesis that can now be based on practically any standard HPLC or LC instrumentation. The new technology offers the following advantages in comparison to that of manual oligosaccharide synthesis on polymer supports: faster reaction times, real-time reaction monitoring using an HPLC detection system, and that all steps and sequences can be automated using standard HPLCmanaging computer software. This work was greatly inspired by and is complementary to other automated approaches19,25,40,41 developed for the synthesis of oligosaccharides and related compounds including oncolumn,42 flow-through,24,43 microfluidic,44 and other related processes.45,46 Further optimization of the HPLCbased technology and its application to other platforms and targets is currently underway. Acknowledgment. This work was supported by awards from the NIGMS (GM090254 and GM077170). Dr. Winter and Mr. Kramer (UM;St. Louis) are thanked for HRMS determinations. Supporting Information Available. Experimental procedures, extended experimental data, 1H and 13C NMR spectra for all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. (40) Sears, P.; Wong, C. H. Science 2001, 291, 2344–2350. (41) Machida, K.; Hirose, Y.; Fuse, S.; Sugawara, T.; Takahashi, T. Chem. Pharm. Bull. 2010, 58, 87–93. (42) Mukhopadhyay, B.; Maurer, S. V.; Rudolph, N.; van Well, R. M.; Russell, D. A.; Field, R. A. J. Org. Chem. 2005, 70, 9059–9062. (43) Baumann, M.; Baxendale, I. R.; Ley, S. V. Mol. Diversity 2011, 15, 613–630. (44) Martin, J. G.; Gupta, M.; Xu, Y.; Akella, S.; Liu, J.; Dordick, J. S.; Linhardt, R. J. J. Am. Chem. Soc. 2009, 131, 11041–11048. (45) Roper, K. A.; Lange, H.; Polyzos, A.; Berry, M. B.; Baxendale, I. R.; Ley, S. V. Beilstein J. Org. Chem. 2011, 7, 1648–1655. (46) Opalka, S. M.; Longstreet, A. R.; McQuade, D. T. Beilstein J. Org. Chem. 2011, 1671–1679. The authors declare no competing financial interest.
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